Large vehicles such as freight hauling trucks with eighteen wheels and inflated rubber tires encounter multiple sources of resistance, which reduce the efficiency with which these vehicles operate. One source of resistance is the rolling resistance between the truck tires and the road surface. A second source of energy loss results from the mechanical friction of internal engine and drive train components. A third source is aerodynamic drag or “windage” losses.
Mechanical friction can dominate other losses at low speeds. For example, when a large truck starts up, the main losses are various friction losses due to the relative motion of the mechanical components of the truck. As the truck speed increases, the power required to overcome the mechanical resistance of engine components, transmission and wheel bearings and tires is roughly proportional to the ground speed, and thus the energy expended is nearly constant after the lubricating oils, seals, bearings and tires reach a steady state temperature condition.
The power required to overcome air resistance increases as a function of the cube of the relative air speed, and the energy expended per mile traveled is proportional to the square of the relative speed. Thus, at speeds above about 30 miles per hour (MPH), overcoming air resistance becomes the largest power requirement for freight delivery vehicles.
Box-like trucks without streamlining (e.g., a typical tractor-trailer truck combination) traveling at 45 MPH require about ⅓ as much power and expend about ½ as much energy per mile as is required to travel at 65 MPH. Increasing the relative air speed of the vehicle, such as traveling against a 25 MPH head wind at a 65 MPH ground speed (to produce a 90 MPH relative air speed), requires about 2.6 times greater power and 1.9 times greater energy expenditure.
Hybrid vehicles, e.g., automobiles with regenerative braking systems that substantially rely upon battery-powered electric propulsion, generally provide greater fuel economy in stop-and-go city driving than at higher air-resistance conditions during highway travel. The opposite result of better highway fuel economy at moderate highway speeds as opposed to start-and-stop driving conditions applies to vehicles without such regenerative braking. Wind resistance above relative speeds of about 55 MPH overcomes the constant velocity advantage, and fuel economy at 70 MPH is generally 25 to 30% less than at 55 MPH for stylized passenger cars with various degrees of streamlining to reduce air resistance.
Several vehicles have achieved greater fuel economy at 65 MPH than at lower speeds. For example, conventional internal combustion engine propulsion systems (non-hybrid drive trains) in vehicles such as the 1994 Oldsmobile Cutlass achieved better fuel economy of 25 MPG at 65 MPH as compared to 23 MPG at 45 MPH. In a similar example, the 1997 Toyota Celica achieved 43.5 MPG at 65 MPH as compared to 42.5 MPG at 55 MPH.
Streamlining “large box” delivery vehicles and heavy freight trucks has taken the form of introducing various degrees of surface rounding to reduce the relative-motion wind forces. Manufacturers and operators, however, generally oppose the loss of cargo packing efficiency and space that rounding the corners of the truck “boxes” requires. At least some conventional components for reducing the air pressure and drag in various locations of the truck have been introduced. Such approaches are cumbersome and have been found to actually increase drag, and/or cause vehicle steering uncertainties, and/or contribute to collisions and rollovers at the onset of some travel conditions, particularly crosswinds.
FIG. 1 shows a conventional approach for adding features to round the tractor surfaces, along with added structures that blend the streamlining of the tractor with the surfaces of the cargo trailer. This provides drag reduction by streamlining the flow the air up and over the freight box. Rounding the vertical edges of the freight box can further reduce drag. However, these efforts to reduce energy losses tend to add curb weight. In addition, streamlining can add cross-sectional area (which increases cross-wind forces), and adds to the initial cost of the vehicle. These factors can result in unattractive recovery projections compared to competing opportunities for improving engine efficiency, reducing maintenance and/or eliminating emission control measures.
Braking represents another area of vehicle operation that creates waste energy. Conventional brakes typically convert kinetic energy to heat, which is then dumped. In other conventional arrangements, “jake braking” uses the back pressure available from the vehicle's engine cylinders to provide braking force by restricting the exhaust. In still further conventional arrangements mechanical/electrical regenerative braking is used to slow the vehicle by driving a generator operably coupled to the vehicle wheels during braking. In any of the foregoing arrangements, the braking processes produce waste energy that reduces the efficiency of the vehicle.
For at least the foregoing reasons, there exists a need in the relevant art for more efficient transport systems, and more efficient use of natural resources.